net/dns: make exit node DNS ask OSConfigurator for backup resolvers

Updates #1713

Change-Id: I7be9dab2b2c03749b4c2d99f9f45c11422ac915a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/debian_resolvconf.go

@@ -18,6 +18,7 @@
 	"path/filepath"
 
 	"tailscale.com/atomicfile"
+	"tailscale.com/types/dnstype"
 	"tailscale.com/types/logger"
 )
 
@@ -173,6 +174,10 @@ func (m *resolvconfManager) GetBaseConfig() (OSConfig, error) {
 	return readResolv(&conf)
 }
 
+func (m *resolvconfManager) GetExitNodeForwardResolver() ([]dnstype.Resolver, error) {
+	return getExitNodeForwardResolverFromBaseConfig(m)
+}
+
 func (m *resolvconfManager) Close() error {
 	if err := m.deleteTailscaleConfig(); err != nil {
 		return err

net/dns/direct.go

@@ -18,6 +18,7 @@
 	"strings"
 
 	"inet.af/netaddr"
+	"tailscale.com/types/dnstype"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/dnsname"
 )
@@ -183,6 +184,24 @@ func (m *directManager) readResolvFile(path string) (OSConfig, error) {
 	return readResolv(bytes.NewReader(b))
 }
 
+func (m *directManager) GetExitNodeForwardResolver() (ret []dnstype.Resolver, retErr error) {
+	for _, filename := range []string{backupConf, resolvConf} {
+		if oc, err := m.readResolvFile(filename); err == nil {
+			for _, ip := range oc.Nameservers {
+				if ip != netaddr.IPv4(100, 100, 100, 100) {
+					ret = append(ret, dnstype.Resolver{Addr: netaddr.IPPortFrom(ip, 53).String()})
+				}
+			}
+			if len(ret) > 0 {
+				return ret, nil
+			}
+		} else if !os.IsNotExist(err) && retErr == nil {
+			retErr = err
+		}
+	}
+	return nil, retErr
+}
+
 // ownedByTailscale reports whether /etc/resolv.conf seems to be a
 // tailscale-managed file.
 func (m *directManager) ownedByTailscale() (bool, error) {

net/dns/manager.go

@@ -62,6 +62,11 @@ func (m *Manager) Set(cfg Config) error {
 	if err != nil {
 		return err
 	}
+	exitNodeBackupResolvers, err := m.os.GetExitNodeForwardResolver()
+	if err != nil {
+		return err
+	}
+	rcfg.ExitNodeBackupResolvers = exitNodeBackupResolvers
 
 	m.logf("Resolvercfg: %v", logger.ArgWriter(func(w *bufio.Writer) {
 		rcfg.WriteToBufioWriter(w)

net/dns/manager_test.go

@@ -45,6 +45,10 @@ func (c *fakeOSConfigurator) GetBaseConfig() (OSConfig, error) {
 	return c.BaseConfig, nil
 }
 
+func (c *fakeOSConfigurator) GetExitNodeForwardResolver() ([]dnstype.Resolver, error) {
+	return getExitNodeForwardResolverFromBaseConfig(c)
+}
+
 func (c *fakeOSConfigurator) Close() error { return nil }
 
 func TestManager(t *testing.T) {
@@ -213,6 +217,7 @@ func TestManager(t *testing.T) {
 				Routes: upstreams(
 					".", "8.8.8.8:53",
 					"corp.com.", "2.2.2.2:53"),
+				ExitNodeBackupResolvers: []dnstype.Resolver{{Addr: "8.8.8.8:53"}},
 			},
 		},
 		{
@@ -249,6 +254,7 @@ func TestManager(t *testing.T) {
 					".", "8.8.8.8:53",
 					"corp.com.", "2.2.2.2:53",
 					"bigco.net.", "3.3.3.3:53"),
+				ExitNodeBackupResolvers: []dnstype.Resolver{{Addr: "8.8.8.8:53"}},
 			},
 		},
 		{
@@ -294,6 +300,7 @@ func TestManager(t *testing.T) {
 					"dave.ts.com.", "1.2.3.4",
 					"bradfitz.ts.com.", "2.3.4.5"),
 				LocalDomains:            fqdns("ts.com."),
+				ExitNodeBackupResolvers: []dnstype.Resolver{{Addr: "8.8.8.8:53"}},
 			},
 		},
 		{
@@ -343,6 +350,7 @@ func TestManager(t *testing.T) {
 					"dave.ts.com.", "1.2.3.4",
 					"bradfitz.ts.com.", "2.3.4.5"),
 				LocalDomains:            fqdns("ts.com."),
+				ExitNodeBackupResolvers: []dnstype.Resolver{{Addr: "8.8.8.8:53"}},
 			},
 		},
 		{

net/dns/manager_windows.go

@@ -17,6 +17,7 @@
 	"golang.org/x/sys/windows/registry"
 	"golang.zx2c4.com/wireguard/windows/tunnel/winipcfg"
 	"inet.af/netaddr"
+	"tailscale.com/types/dnstype"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/dnsname"
 )
@@ -340,6 +341,10 @@ func (m windowsManager) GetBaseConfig() (OSConfig, error) {
 	}, nil
 }
 
+func (m windowsManager) GetExitNodeForwardResolver() ([]dnstype.Resolver, error) {
+	return getExitNodeForwardResolverFromBaseConfig(m)
+}
+
 // getBasePrimaryResolver returns a guess of the non-Tailscale primary
 // resolver on the system.
 // It's used on Windows 7 to emulate split DNS by trying to figure out

net/dns/nm.go

@@ -16,6 +16,7 @@
 	"github.com/godbus/dbus/v5"
 	"inet.af/netaddr"
 	"tailscale.com/net/interfaces"
+	"tailscale.com/types/dnstype"
 	"tailscale.com/util/dnsname"
 	"tailscale.com/util/endian"
 )
@@ -374,6 +375,10 @@ type dnsPrio struct {
 	return ret, nil
 }
 
+func (m *nmManager) GetExitNodeForwardResolver() ([]dnstype.Resolver, error) {
+	return getExitNodeForwardResolverFromBaseConfig(m)
+}
+
 func (m *nmManager) Close() error {
 	// No need to do anything on close, NetworkManager will delete our
 	// settings when the tailscale interface goes away.

net/dns/noop.go

@@ -4,14 +4,21 @@
 
 package dns
 
+import "tailscale.com/types/dnstype"
+
 type noopManager struct{}
 
+var _ OSConfigurator = noopManager{}
+
 func (m noopManager) SetDNS(OSConfig) error  { return nil }
 func (m noopManager) SupportsSplitDNS() bool { return false }
 func (m noopManager) Close() error           { return nil }
 func (m noopManager) GetBaseConfig() (OSConfig, error) {
 	return OSConfig{}, ErrGetBaseConfigNotSupported
 }
+func (m noopManager) GetExitNodeForwardResolver() ([]dnstype.Resolver, error) {
+	return nil, nil
+}
 
 func NewNoopManager() (noopManager, error) {
 	return noopManager{}, nil

net/dns/openresolv.go

@@ -12,6 +12,8 @@
 	"fmt"
 	"os/exec"
 	"strings"
+
+	"tailscale.com/types/dnstype"
 )
 
 // openresolvManager manages DNS configuration using the openresolv
@@ -90,6 +92,10 @@ func (m openresolvManager) GetBaseConfig() (OSConfig, error) {
 	return readResolv(&buf)
 }
 
+func (m openresolvManager) GetExitNodeForwardResolver() ([]dnstype.Resolver, error) {
+	return getExitNodeForwardResolverFromBaseConfig(m)
+}
+
 func (m openresolvManager) Close() error {
 	return m.deleteTailscaleConfig()
 }

net/dns/osconfig.go

@@ -8,6 +8,7 @@
 	"errors"
 
 	"inet.af/netaddr"
+	"tailscale.com/types/dnstype"
 	"tailscale.com/util/dnsname"
 )
 
@@ -18,20 +19,35 @@ type OSConfigurator interface {
 	// configuration is removed.
 	// SetDNS must not be called after Close.
 	SetDNS(cfg OSConfig) error
+
 	// SupportsSplitDNS reports whether the configurator is capable of
 	// installing a resolver only for specific DNS suffixes. If false,
 	// the configurator can only set a global resolver.
 	SupportsSplitDNS() bool
+
 	// GetBaseConfig returns the OS's "base" configuration, i.e. the
 	// resolver settings the OS would use without Tailscale
 	// contributing any configuration.
 	// GetBaseConfig must return the tailscale-free base config even
 	// after SetDNS has been called to set a Tailscale configuration.
 	// Only works when SupportsSplitDNS=false.
-
+	//
 	// Implementations that don't support getting the base config must
 	// return ErrGetBaseConfigNotSupported.
 	GetBaseConfig() (OSConfig, error)
+
+	// GetExitNodeForwardResolver returns the resolver(s) that should
+	// be used as a fallback for the exit node's DNS-over-HTTP peerapi
+	// to send DNS queries from peers on to, in the case where the tailnet
+	// doesn't have global DNS servers configured.
+	//
+	// For example, on Linux with systemd-resolved, this will
+	// return 127.0.0.53:53.
+	//
+	// On other systems, it'll usually be the value of
+	// GetBaseConfig.Nameservers.
+	GetExitNodeForwardResolver() ([]dnstype.Resolver, error)
+
 	// Close removes Tailscale-related DNS configuration from the OS.
 	Close() error
 }
@@ -90,3 +106,16 @@ func (a OSConfig) Equal(b OSConfig) bool {
 // OSConfigurator.GetBaseConfig returns when the OSConfigurator
 // doesn't support reading the underlying configuration out of the OS.
 var ErrGetBaseConfigNotSupported = errors.New("getting OS base config is not supported")
+
+func getExitNodeForwardResolverFromBaseConfig(o OSConfigurator) (ret []dnstype.Resolver, retErr error) {
+	oc, err := o.GetBaseConfig()
+	if err != nil {
+		return nil, err
+	}
+	for _, ip := range oc.Nameservers {
+		if ip != netaddr.IPv4(100, 100, 100, 100) {
+			ret = append(ret, dnstype.Resolver{Addr: netaddr.IPPortFrom(ip, 53).String()})
+		}
+	}
+	return ret, nil
+}

net/dns/resolved.go

@@ -19,6 +19,7 @@
 	"golang.org/x/sys/unix"
 	"inet.af/netaddr"
 	"tailscale.com/health"
+	"tailscale.com/types/dnstype"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/dnsname"
 )
@@ -332,6 +333,10 @@ func (m *resolvedManager) GetBaseConfig() (OSConfig, error) {
 	return OSConfig{}, ErrGetBaseConfigNotSupported
 }
 
+func (m *resolvedManager) GetExitNodeForwardResolver() ([]dnstype.Resolver, error) {
+	return []dnstype.Resolver{{Addr: "127.0.0.53:53"}}, nil
+}
+
 func (m *resolvedManager) Close() error {
 	m.cancelSyncer()
 

net/dns/resolver/tsdns.go

@@ -83,6 +83,14 @@ type Config struct {
 	// LocalDomains is a list of DNS name suffixes that should not be
 	// routed to upstream resolvers.
 	LocalDomains []dnsname.FQDN
+	// ExitNodeBackupResolvers are where the local node when
+	// acting as an exit node and serving a DNS proxy should
+	// forward DNS requests to in the case where there are no
+	// routes found. For example, for Linux systemd-resolved
+	// machines this is likely 127.0.0.53:53.
+	// If it's empty, there are no backups and the OS should
+	// be queried directly using its OS-level DNS APIs.
+	ExitNodeBackupResolvers []dnstype.Resolver
 }
 
 // WriteToBufioWriter write a debug version of c for logs to w, omitting
@@ -206,6 +214,7 @@ type Resolver struct {
 	localDomains            []dnsname.FQDN
 	hostToIP                map[dnsname.FQDN][]netaddr.IP
 	ipToHost                map[netaddr.IP]dnsname.FQDN
+	exitNodeBackupResolvers []dnstype.Resolver
 }
 
 type ForwardLinkSelector interface {
@@ -253,9 +262,16 @@ func (r *Resolver) SetConfig(cfg Config) error {
 	r.localDomains = cfg.LocalDomains
 	r.hostToIP = cfg.Hosts
 	r.ipToHost = reverse
+	r.exitNodeBackupResolvers = append([]dnstype.Resolver(nil), cfg.ExitNodeBackupResolvers...)
 	return nil
 }
 
+func (r *Resolver) exitNodeForwardResolvers() []dnstype.Resolver {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	return r.exitNodeBackupResolvers
+}
+
 // Close shuts down the resolver and ensures poll goroutines have exited.
 // The Resolver cannot be used again after Close is called.
 func (r *Resolver) Close() {
@@ -312,34 +328,13 @@ func (r *Resolver) HandleExitNodeDNSQuery(ctx context.Context, q []byte, from ne
 
 	err = r.forwarder.forwardWithDestChan(ctx, packet{q, from}, ch)
 	if err == errNoUpstreams {
-		// Handle to the system resolver.
+		backup := r.exitNodeForwardResolvers()
-		switch runtime.GOOS {
+		if len(backup) > 0 {
-		case "linux":
+			var extra []resolverAndDelay
-			// Assume for now that we don't have an upstream because
+			for _, v := range backup {
-			// they're using systemd-resolved and we're in Split DNS mode
+				extra = append(extra, resolverAndDelay{name: v})
-			// where we don't know the base config.
+			}
-			//
+			err = r.forwarder.forwardWithDestChan(ctx, packet{q, from}, ch, extra...)
-			// TODO(bradfitz): this is a lazy assumption. Do better, and
-			// maybe move the HandleExitNodeDNSQuery method to the dns.Manager
-			// instead? But this works for now.
-			err = r.forwarder.forwardWithDestChan(ctx, packet{q, from}, ch, resolverAndDelay{
-				name: dnstype.Resolver{
-					Addr: "127.0.0.1:53",
-				},
-			})
-		default:
-			// TODO(bradfitz): if we're on an exit node
-			// on, say, Windows, we need to parse the DNS
-			// packet in q and call OS-native APIs for
-			// each question. But we'll want to strip out
-			// questions for MagicDNS names probably, so
-			// they don't loop back into
-			// 100.100.100.100. We don't want to resolve
-			// MagicDNS names across Tailnets once we
-			// permit sharing exit nodes.
-			//
-			// For now, just return an error.
-			return nil, err
 		}
 	}
 	if err != nil {